This invention relates to the production of synthesis gas for use for the synthesis of hydrogen-containing compounds such as ammonia or alcohols, e.g. methanol, and to the synthesis of such hydrogen-containing compounds from the synthesis gas.
Such hydrogen-containing compounds are usually synthesised in a synthesis loop wherein a mixture of fresh synthesis gas, termed make-up gas, and recycle gas, is fed at elevated temperature and pressure to a synthesis reactor containing a suitable catalyst for the synthesis reaction. The desired hydrogen-containing compound is then separated from the reacted gas leaving the synthesis reactor, for example by cooling the reacted synthesis gas to condense the synthesised hydrogen-containing compound as a liquid phase which can readily be separated. The gas remaining after separation of the desired hydrogen-containing compound is then recycled to the synthesis reactor as the recycle gas. Since the make-up gas often contains components that are inert in the synthesis reaction and/or an excess of one of the reactants, part of the gas is taken from the loop as a purge to avoid a build-up of inerts, or the reactant that is in an excess, in the gas circulating round the synthesis loop. Often some or all of the purge is subjected to a purification process to recover desired reactants which are recycled, directly to the synthesis as part of the recycle gas, or to a suitable point in the production of the make-up gas. Where the process used to produce the make-up gas includes a separation step effective to remove undesired components, e.g. the excess of one reactant or inerts, the purge can be recycled to a point in the production of the make-up gas upstream of that separation step.
The make-up gas is often produced by a series of steps including steam reforming of a hydrocarbon feedstock, particularly natural gas or naphtha. In this steam reforming stage the feedstock, usually after desulphurisation, is reacted at elevated temperature and pressure with steam, and sometimes also carbon dioxide, over a steam reforming catalyst, commonly nickel supported on a refractory material such as alumina or calcium aluminate, to give a gas stream containing hydrogen, carbon oxides and methane. The reforming catalyst is normally disposed in tubes heated in a furnace fired by a suitable fuel. Some or all of the aforesaid purge gas may be used as at least part of the furnace fuel.
Often, particularly where the process is used to produce ammonia synthesis gas, the primary reformed gas is subjected to a partial oxidation step, often called secondary reforming, wherein the primary reformed reformed gas is partially oxidised with a gas containing free oxygen, e.g. oxygen itself, or air (or oxygen-enriched or oxygen-depleted air) where it is desired to introduce nitrogen into the make-up gas, for example for ammonia synthesis gas. In this secondary reforming step, the partially oxidised, i.e. partially combusted, gas usually is then passed through a steam reforming catalyst to effect further reforming to decrease the methane content. Such a step of partial oxidation followed by passage through a steam reforming catalyst is often termed autothermal reforming. The heat required for the endothermic reforming reaction is thus provided by the heat evolved in the partial combustion. Depending on the intended use, the resultant product gas, i.e. primary reformed gas, or secondary reformed gas where such a partial oxidation step is used, is further treated to give the make-up gas. The further treatment will depend on the intended use.
For ammonia synthesis, the make-up gas is required to contain hydrogen and nitrogen. The secondary reformed gas obtained using air (or oxygen-enriched or oxygen-depleted air) as the oxygen-containing gas will contain hydrogen, nitrogen, carbon oxides, methane and argon. Thus for ammonia synthesis gas the secondary reformed gas is usually subjected to one or more steps of the shift reaction with steam to convert carbon monoxide to carbon dioxide with the production of more hydrogen, and then carbon dioxide and water vapour are removed. Since carbon oxides act as poisons for ammonia synthesis catalysts, the residual carbon oxides are usually removed, for example by methanation. Alternatively, the shifted gas may be subjected to a catalytic selective oxidation to convert the residual carbon monoxide to carbon dioxide and then the carbon dioxide and water vapour removed. Since hydrogen and nitrogen react in the proportions of 3 moles of hydrogen to each mole of nitrogen to produce ammonia, the make-up gas desirably has a hydrogen/nitrogen molar ratio of about 3. While this can be achieved by choosing the primary and secondary reforming conditions so that the amount of air employed is that which introduces the desired amount of nitrogen, in order to reduce the amount of reforming that has to be effected in the primary reformer, the amount of air employed in the secondary reforming step is often such that the autothermally reformed gas contains an excess of nitrogen over that required for ammonia synthesis. Consequently in such cases there will usually be a step of nitrogen removal, either on the make-up gas prior to its addition to the synthesis loop, or on a stream taken from the loop: in the latter case, the excess of nitrogen is separated from the stream taken from the loop, leaving a stream enriched in hydrogen. This hydrogen-enriched stream is then returned to the loop. The separation of the excess of nitrogen also often serves to remove some or all of the residual methane and argon (which act as inerts in the ammonia synthesis process). The resultant gas waste gas stream containing the excess of nitrogen and residual methane thus has some fuel value and so is often used as part, or all, of the fuel employed to heat the primary reformer.
For synthesis of oxygen-containing organic compounds such as methanol, the make-up gas contains hydrogen, carbon monoxide and carbon dioxide. The parameter "R", given by the equation EQU R=([H.sub.2 ]-[CO.sub.2 ])/([CO]+[CO.sub.2 ])
where [H.sub.2 ], [CO], and [CO.sub.2 ] represent the molar proportions of hydrogen, carbon monoxide and carbon dioxide respectively, is often used in relation to the composition of the make-up gas. A make up gas having a value of "R" equal to 2 has the stoichiometric composition for methanol synthesis.
While a secondary reforming step is often not employed in the manufacture of methanol synthesis gas, its use may enable the synthesis gas to have a composition more suited to methanol synthesis. Thus, in the absence of a secondary reforming step, assuming the feedstock is natural gas, the synthesis gas will contain more hydrogen than is required to convert the carbon oxides present to methanol, i.e. "R" will be well above 2. The use of a secondary reforming step enables the value of "R" to be decreased to a suitable level, e.g. in the range 1.8 to 2.2. Thus it has been proposed in GB-A-2099846 to operate the primary reforming stage at pressures in the range 35 to 55 bar abs., using lower outlet temperatures than is conventional, to give a gas stream containing a relatively high methane content and then to subject this primary reformed gas to secondary reforming with oxygen.
For synthesis gas to be used for the manufacture of oxygenated organic compounds such as methanol, the reformed gas, after secondary reforming (if such a step is employed), may need no further treatment except cooling and removal of water vapour.
The aforesaid primary reforming step employing catalyst-containing tubes heated in a fired furnace is not very efficient thermally and involves large and costly installations. There have been various proposals for decreasing the duty of primary reformers, e.g. by partially bypassing the primary reformer so that part of the feed is fed directly to the secondary reformer. Thus in order to increase the throughput of existing plants, it has been proposed in e.g. GB-A-2160516 to provide a partial bypass of the primary reformer so that some of the feedstock is fed directly to the secondary reformer. A similar process is described in GB-A-1569014. Also the bypassing of the primary reformer means that the overall steam ratio can be decreased so that the volume of gas that has to be cooled, per volume of carbon oxides produced, is less.
These processes however present some difficulties as it is necessary to mix the relatively cold feedstock bypassing the primary reformer with the hot primary reformed gas, and/or to design the secondary reformer with the provision of a separate, additional, feed thereto. The provision of such a separate additional feed presents mixing problems while the addition of the bypass feedstock to the hot primary reformed gas presents problems particularly where it may be desirable to isolate the bypass stream while maintaining the primary and secondary reforming stages in operation.
This isolation ability is particularly desirable where, as suggested in the aforementioned GB-A-2160516 the feedstock is liquid at room temperature, e.g. naphtha, and the feedstock bypassing the primary reformer is subjected to an adiabatic catalytic reaction with steam to produce a gas containing methane as essentially the major hydrocarbon component. Such an adiabatic process, which is herein termed a pre-reforming process, is desirable in order to avoid the carbon deposition which is liable to occur through thermal cracking of hydrocarbons of higher molecular weight than methane when the bypass gas is mixed with the hot product from the primary reformer. Unfortunately the life of the catalyst employed in such an adiabatic pre-reforming process is generally far less than that of the primary or secondary reforming catalysts and so the pre-reforming catalyst will require changing far more frequently than the primary or secondary reforming catalyst. It is therefore desirable to provide for the pre-reforming catalyst to be changed without shutting down the primary and secondary reformers, and so in the aforementioned arrangement wherein the bypass gas, after the pre-reforming stage, is mixed with the hot primary reformed gas, or is fed directly to the secondary reformer, some valve means capable of operating at high temperatures is necessary to effect that isolation.